The present invention relates to optical devices that are based on light transmission in planar waveguides, and more particularly, to such devices employing the use of non-linear organic polymers having photovoltaic and piezoelectric interfaces.
Integrated optics is concerned with dielectric structures that confine the propagating light to a region with one or two very small dimensions, on the order of the optical wavelength. An integrated optical circuit can include lasers, integrated lenses, switches, interferometers, polarizers, modulators, detectors, etc. Important uses for integrated optical circuits include signal processing and optical communications. Integrated optical circuits can be used in such systems as optical transmitters, switches, repeaters, and receivers.
The advantages of having an optical system in the form of an integrated optical circuit rather than a conventional series of components include (apart from miniaturization) reduced sensitivity to air currents and to mechanical vibrations of the separately mounted parts, low driving voltages and high efficiency, robustness, and reproducibility and economy. As in the case of semiconductor integrated circuits, an integrated optical circuit might be fabricated on or just within the surface of one material (the substrate) modified for the different components by shaping structures (using etching, for example) or incorporating suitable substitutes or dopants, or alternatively, by depositing or expitaxially growing additional layers. It is also possible to construct independent components which are then attached to from the integrated optical circuit. This option, called hybrid, has the advantage that each component could be optimized, for example, by using gallium aluminum arsenide lasers as sources for an integrated optical circuit and silicon detectors. In the former case, the integrated optical circuit is called monolithic, and is expected to have the advantage of ease of processing, similar to the situation for monolithic semiconductor integrated circuits. One of the most promising materials for monolithic integrated optical circuits are direct band-gap semiconductors composed of III-V materials such as gallium aluminum arsenide [(GaAl)As] and indium gallium arsenide phosphide [(InGa)(AsP)] since with suitable processing they ma perform almost all necessary operations as lasers, switches, modulators, detectors, and so forth.
In the prior art, the simplest optical waveguide is a three-layer or sandwich structure with the index of refraction largest in the middle or waveguiding layer. The lower and top layers are usually the substrate and superstrate, respectively. Often, the top layer is air and the waveguide layer is referred to as a film. Sometimes, too, the outer regions are called cladding layers. A guided wave does not have light distributed uniformly across the waveguide, but is a pattern that depends on the indices of refraction of all three layers and the guide thickness. The waveguide is usually designed by selecting its refractive index and thickness, so that only one such characteristic pattern propagates with no change in shape. This pattern, referred to as the fundamental or lowest-order mode, travels down the guide with a characteristic velocity.
Waveguides have been made of many different materials, most of which may be categorized as ferroelectric, semiconductor, or amorphous. Examples of these classes are lithium niobate, gallium arsenide/gallium aluminum arsenide [GaAs/(GaAl)As], and glass, respectively. Methods for fabricating a waveguide layer at the surface of lithium niobate include heating the crystal in a vacuum to drive off lithium oxide or diffusing titanium metal into the crystal. Both processes create a region of high refractive index near the surface; air is the superstrate. A semiconductor waveguide is fabricated, for example, by growing successively crystalline layers of (Ga.sub.0.7 Al.sub.0.3)As, GaAs, and (Ga.sub.0.7 Al.sub.0.3)As. The thin GaAs waveguide layer of high refractive index is thus interposed between thicker cladding regions of the lower-index (Ga.sub.0.7 Al.sub.0.3)As. Glass waveguides may be formed, for example, by sputter deposition of a relatively high-refractive-index glass on a lower-index glass substrate.
Waveguides that confine light in two dimensions, rather than one, utilize refractive index differences in both transverse directions. Examples are the rib guide and the titanium-diffused channel guide. In fabricating a rib guide, photolithography is employed to delineate the stripe, followed by chemical or dry etching to remove the undesired material. The channel guide is produced by etching away all but a strip of metal prior to diffusion.
An external light beam may be coupled into a waveguide by introducing the light at the end of the guide at an edge of the substrate or through the surface of the waveguide. The former approach may employ a lens to focus the light beam onto the guide end. Alternatively, the laser or an optical fiber is placed against or in close proximity to the guide end, which is referred to as butt coupling. Light may also be injected through the guide surface with an auxiliary element such as a high-refractive index prism. The angle of light incident on the surface of the waveguide layer can also be modified so as to coincide with the internal angle of the propagating wave by a periodic structure or grating on the waveguide surface. Such a grating may be made by a multiple-step process which begins by depositing a light sensitive material called photoresist and concludes with etching and cleaning. The grating also functions as an output coupler.
The diode laser is already an integrated optics device in a sense since the lasing medium as a waveguide laser interposed between two cladding layers. The waveguide layer may be GaAs, in which case the outer regions are composed of (Ga.sub.0.7 Al.sub.0.3)As, for example. When used as a separate source, the crystal facets act as end reflectors. The diode laser's high efficiency, low-voltage operation, small size, and physical integrity cause it to be the laser of choice in many hybrid integrated optics applications. Moreover, it lends itself to integration in an integrated optical circuit where reflectivity may be provided by introducing an appropriate periodic structure. This could be a thickness variation in the waveguide layer. Devices utilizing such structures are called distributed feedback or distributed Bragg reflector lasers.
Both lithium niobate and gallium arsenide belong to the family of electro-optically active crystals. When an electric field is applied to these materials, their refractive indices are modified. This effect is employed in prior art integrated optical circuit switching and modulation applications. To construct a switch in the prior art, gold or other conducting electrodes are deposited on a lithium niobate integrated optical circuit surface parallel to two closely spaced waveguides. If the electrodes and waveguides are suitably designed, the applications of specific small voltages to the electrodes will cause the transfer of optical power from one waveguide to its neighbor with high efficiency and little residual power in the initial guide.